In making geophysical surveys and in deployment of various electronic surveillance and detection devices which interface with the ground and in the selection of antenna sites for communication systems, an accurate determination of the resistivity -- or its reciprocal -- the conductivity -- of localized samples of soil is often necessary.
Although there are several reliable laboratory techniques for measurement of the electromagnetic parameters of lossy dielectric media, they are, by nature of their instrumentation, wholly unsuited for use outside of the laboratory. Devices that are designed for use in the field, such as those employing the wave-tilt or polarization anomaly technique, lack the selectivity of the sample measuring devices of the laboratory. Both the wave-tilt and polarization anomaly techniques are integrating methods which measure the bulk conductivity of the earth. In these methods, there is some inherent weighting of the measurements toward the material directly beneath the device; however, the data is strongly influenced by all of the surrounding earth, as well as by local topography. Several methods of measuring the characteristics of soil use an array of probes placed in situ in the soil and thus involve an unconfined soil sample. With such a large unconfined sample one has to deal with a so-called infinite halfspace, roughly the shape of a hemisphere for a single probe and more or less dumb-bell shaped for two probes. Not only are the current paths involved in such an infinite halfspace many and complex, but departure from homogeneity in this halfspace (practically always the case) will distort the equivalent space and cause severe errors in interpretation. Furthermore, measurements for such an unconfined sample can only be a mean value not at all representative of localized regions within that relatively large halfspace.
Obviously, the integrating features of these techniques would be of some value where the gross conductivity of the earth is of importance. However, these devices would be equally ill-suited for the task of sorting out the individual characteristics of each soil occurring within a particular site. It is also important to note that these latter devices are generally restricted to measurements at frequencies in the low megahertz range and that a high to medium power signal source is required. Both of these features tend to limit application of these techniques to the communications field.